The present invention relates generally to electric motors and, more particularly, to controlling cogging in electric motors.
Many electromagnetic machines in general, and permanent magnet electric motors in particular, exhibit torque irregularities as the rotor rotates with respect to the stator. Such irregularities produce non-uniform torque output and, thus, non-uniform rotation of the rotor. These torque irregularities may be caused by the physical construction of a given machine, They can result from, for example, a bearing that sticks in a given rotor position or the fact that, because of the electromagnetic characteristics of the machine, the rotor tends to prefer certain angular positions with respect to the stator. Torque irregularities resulting from the electromagnetic characteristics of a permanent-magnet machine are commonly known as torque ripple, and the component that is present even when the stator windings are not energized is known as xe2x80x9ccoggingxe2x80x9d.
Because cogging is generally undesirable for certain electric motor applications, including automotive power steering applications, techniques have been explored to reduce cogging. For example, optimization techniques have been applied to identify magnet pole geometries that yield reduced cogging. As recognized by the inventor hereof, however, these known techniques do not predict cogging in an electric motor as a function of the magnet pole geometry, and therefore do not facilitate the selection of design parameter values which yield a desired level of cogging in the electric motor,
The inventor hereof has succeeded at designing a method of determining magnet pole geometry for controlling cogging in an electric motor. The method includes defining the magnet pole geometry in terms of at least one design parameter, predicting cogging as a function of the design parameter, and, from the predicted cogging, selecting a value for the design parameter which corresponds to a desired level of cogging in the electric motor.